Abstract Anxiety disorders are the most prevalent form of pediatric psychopathology creating a significant burden on individuals and society. The overall goal of the funded R01 parent grant awarded to Dr. Nathan Fox is designed to examine the developmental pathways from early fearful temperament, Behavioral Inhibition (BI), to anxiety emerging during late adolescence. The parent grant examines how specific internal (e.g., reactive vs proactive cognitive control) and external (e.g., supportive friendships) factors, assessed during mid- adolescence (age 15), interact with BI to exacerbate or mitigate the development of anxiety by late adolescence (age 18) within a large sample of adolescents. However, the parent grant did not plan to (1) collect naturalistic measures of social interactions and anxiety, and (2) examine social rejection, a social scenario that may be particularly important for children at risk for anxiety because of BI. Accordingly, in this supplement to the parent grant, we propose to integrate ecological momentary assessment (EMA) methods to measure the real-world experiences of anxiety and neural measures of social rejection during a crucial developmental period, the transition to young adulthood. In Aim 1 of this grant, the candidate will investigate the mediating role of neural reactivity to social exclusion right after high-school graduation on the association between BI and real-world social anxiety outcomes during the transition to young adulthood. We hypothesize that the longitudinal relation between BI and social outcomes during the transition to young adulthood will be mediated by greater neural responses to social exclusion as indexed by increased frontal theta power. In Aim 2 of this grant, the candidate will utilize novel intensive sampling methods to examine the moderating role of BI on the effects of social context on the momentary experience of anxiety during the transition to young adulthood. We predict that the momentary effects of the social context on anxiety will be moderated by BI, such that the effects of the social context on anxiety will be larger for individuals high in BI. Specifically, individuals high in BI will display greater reductions in anxiety in the company of close companions (e.g., close friends or significant other) compared to when being alone or with distant others (e.g., strangers, co-workers, or acquaintances). For individuals low in BI, this difference will be less pronounced. This training project will provide a better understanding of the neural and contextual processes that lead to anxiety and aid the development of prevention and intervention strategies. Additionally, the training project will enhance Dr. Morales? career development by providing research training on advanced EEG and EMA methods. Ultimately, this training project will help accelerate Dr. Morales? transition to research independence.